Denise Gladue Seith, 30, who had worked on Capitol Hill and lived in Northern Virginia for the past 11 years, died of cancer June 10 at George Washington University Hospital.

She had been a legislative assistant to Sen. Mark Andrews (R-N.D.) since he took office in January 1981. Before that, Mrs. Seith had worked for Andrews with the House Appropriations Committee while Andrews was a member of the House of Representatives.

Mrs. Seith was born in Manitoba, Canada, and was reared in Neche, N.D. She was a 1970 graduate of the North Dakota State School of Science. She moved here in 1970. She was a resident of Arlington until moving to Alexandria two years ago.

She was a member of Christ Episcopal Church in Alexandria. She was a former secretary of the North Dakota State Society.

Survivors include her husband, Michael, of Alexandria; her mother, Helen Gladue, and a brother, Daryl Gladue, both of Neche; two sisters, Judy McCann, of Grand Forks, N.D., and Aurelia Bjornstad, of Leroy, N.D.

The family suggests that expressions of sympathy be in the form of contributions to the Memorial Fund, Christ Episcopal Church, Alexandria, Va., or to the American Cancer Society.